The present invention generally relates to a semiconductor light-emitting unit including a semiconductor light-emitting diode and also relates to optical head apparatus and optical disk system including the light-emitting unit. More particularly, the present invention relates to measures to be taken to ensure good emission characteristics for the light-emitting diode.
Recently, various types of optical or magneto-optical disk systems have rapidly increased their demand and expanded their applications. These systems are used for optically writing, reading and erasing information onto/from a high-density, large-capacity optical or magneto-optical storage medium like optical disk or optical card using an optical memory. As for optical disks with a pit pattern, for example, digital audio disks, videodisks, document file disks and disks with data files have found broader and broader applications.
To write or read information onto/from a storage medium like an optical disk, an optical system is needed to make a light beam, which has been produced by a semiconductor light-emitting diode and then condensed to a spot of a very small size, incident on the storage medium. That is to say, to write or read the information onto/from the storage medium successfully and accurately, the optical system as a whole should be constructed with very high precision before everything else. For example, basic functions of an optical head apparatus, which is a key section of an optical disk system, are roughly classified into: condensing the light beam to a very small spot down to the limit of diffraction; performing focus and tracking controls over the optical system; and detecting a pit signal. These functions are implementable by appropriately combining various types of optical systems and photoelectric conversion methods depending on the intended applications. Examples of these combinations include: a semiconductor light-emitting unit including a semiconductor laser diode and a photodetector as a unit; an optical head apparatus (or an optical pickup) including the light-emitting unit, prism and lens; and an optical disk system (or optical disk drive) including the optical head apparatus and a storage medium.
As for an optical disk system applicable to compact discs (CDs), a semiconductor laser diode, operating at a wavelength of around 800 nm belonging to the so-called xe2x80x9cinfrared regionxe2x80x9d of the spectrum, has often been used as its light source. Recently, however, remarkable progress has been achieved in the design technologies for optical systems and semiconductor laser diodes, operating at an even shorter wavelength and yet producing far higher output power, have been developed one after another. As a result, the storage capacity of optical disks has also been increasing year after year.
Also, to provide a downsized and more reliable optical disk system or optical head apparatus at a lower cost, an optical system for an optical head apparatus is simplified by using a hologram according to a proposed technique. Such a technique is disclosed by Wai-Hon Lee in xe2x80x9cHolographic Optical Head for Compact Disc Applicationsxe2x80x9d, Optical Engineering Vol. 28, No. 6, pp. 650-653 (1989).
In the optical disk systems like these, automatic power control (APC) is usually carried out to keep the intensity of radiation emitted from the semiconductor laser diode constant such that write and read operations can be performed even more stably and reproducibly using the semiconductor laser diode.
However, the known optical disk systems have a problem in the relationship between the amount of current I injected into a semiconductor laser diode and the intensity L of radiation emitted from the laser diode, which will be herein called xe2x80x9cI-L characteristicxe2x80x9d.
FIG. 12 illustrates the I-L characteristics using the operating temperature of a semiconductor laser diode as a parameter. As shown in FIG. 12, the operating temperatures are defined by the three ambient temperatures of 0, 30 and 80xc2x0 C. in the illustrated example. In FIG. 12, at room temperature (i.e., 30xc2x0 C.), the radiation starts to be emitted from the semiconductor laser diode at a predetermined current value (i.e., a threshold current value). Then, a substantially linear relationship will be maintained between the current and the emission intensity until the intensity reaches a predetermined value. In contrast, at an elevated temperature (i.e., 80xc2x0 C.), the amount of current needed to start the emission and the quantity of heat generated both increase compared to the room-temperature result. Accordingly, if a high intensity should be attained at such an elevated temperature, then the resultant I-L characteristic cannot be linear anymore. That is to say the gain of the emission with respect to the current supplied decreases. Such a bend of the I-L characteristic is curve, i.e., variation in differentiated intensity of emission with respect to the current value, will be herein called a xe2x80x9ckinkxe2x80x9d of a high-temperature I-L characteristic.
If a semiconductor laser diode is operated under such conditions as causing the kink in the high-temperature I-L characteristic, then the APC control over an optical disk system including the laser diode will lose its stability. Also, if even higher output power should be attained (i.e., if the emission to be attained corresponds to a point on the high-temperature I-L characteristic curve which is even greater than the point where the kink is caused), the far field pattern (FFP) of the emission might be out of order, thus possibly deteriorating the convergence. Accordingly, the semiconductor laser diode should be operated under such conditions as eliminating the kinks from the high-temperature I-L characteristic and yet required emission intensity should be attained. However, to increase the emission intensity of the semiconductor laser diode, the amount of the current injected should be increased. Then, the operating temperature of the semiconductor laser diode rises from the ambient temperature, and therefore, the kink is more likely to be caused in the high-temperature I-L characteristic.
To solve this problem, a known semiconductor laser diode for optical disk system has its structure or material modified such that a greater amount of current can be supplied thereto at room temperature to attain the emission intensity exceeding that corresponding to the point where the kink is caused in the high-temperature I-L characteristic. For example, the amount of current supplied may be defined at the point A shown in FIG. 12. Another known semiconductor laser diode has its heat dissipation ability improved to minimize the temperature rise resulting from the injection of an increased amount of current.
However, if that type of semiconductor laser diode, which has its heat dissipation ability improved to eliminate kinks from the high-temperature I-L characteristic, is operated at a low ambient temperature, then a kink is caused in the low-temperature I-L characteristic when a large amount of current is supplied. For example, if the amount of current supplied is increased at a low temperature (e.g., 0xc2x0 C. shown in FIG. 12), then a kink is observable in the I-L characteristic curve. Accordingly, if a known optical disk system, which has had its heat dissipation ability improved to eliminate the kinks from the high-temperature I-L characteristic, is used at a low temperature (e.g., 10xc2x0 C. or less), then the kink is likely to be caused in the low-temperature I-L characteristic, thus possibly deteriorating its emission performance.
An object of the present invention is providing (1) a semiconductor light-emitting unit that can attain a high emission intensity in a broad temperature range by ensuring good heat dissipation ability for a semiconductor laser diode and by eliminating kinks from its low-temperature I-L characteristic, and (2) optical head apparatus and optical disk system including that unit.
An inventive semiconductor light-emitting unit includes: a semiconductor light-emitting diode; a sub-mount for mounting the diode thereon; and a heating member, incorporated with the sub-mount, for heating the diode.
In the inventive structure, the semiconductor light-emitting diode is heated by the heating member provided for the sub-mount along with the diode. Thus, the diode can be heated effectively, and therefore, kinks can be eliminated from the low-temperature I-L characteristic of the diode even if the ambient temperature is low.
In one embodiment of the present invention, the heating member preferably heats the diode up to such a temperature as substantially eliminating kinks from a low-temperature I-L characteristic of the diode.
In another embodiment of the present invention, the heating member may be a resistor. Then, the light-emitting diode can be heated using a simple structure, thus realizing a semiconductor light-emitting unit with high optical output power at a low cost.
In still another embodiment, the heating member is preferably located closely to the diode on the sub-mount.
In yet another embodiment, the sub-mount may be a semiconductor substrate, and the heating member may be a doped region defined inside the substrate. In such an embodiment, the heating member can be provided easily at a low cost by using the substrate.
In this particular embodiment, the heating member is preferably located below the diode, because the diode can be heated even more effectively in such an arrangement.
More preferably, the unit further includes an insulating layer and a heat-dissipating conductor layer that are formed between the heating member and the diode. In such an embodiment, the temperature of the light-emitting diode is arbitrarily controllable either by heating it or dissipating heat from it. Thus, almost no kinks are observable in the high- or low-temperature I-L characteristic of the diode.
In this case, the thickness of the insulating layer is preferably in the range from 0.1 xcexcm to 1.5 xcexcm, both inclusive, while the thickness of the conductor layer is preferably in the range from 10 xcexcm to 20 xcexcm, both inclusive. Then, the heat can be dissipated or generated highly efficiently.
In still another embodiment, the sub-mount may be the semiconductor substrate with light-receiving areas and may function as a photodetector. In such an arrangement, the light-receiving areas and the heating region can be integrated in a single semiconductor substrate.
In this particular embodiment, the sub-mount preferably has a recess, which is formed by removing part of the substrate from the upper surface thereof. The light-receiving areas are preferably formed on the upper surface of the substrate. And the doped region is preferably defined under the bottom of the recess. In such an embodiment, the light-emitting diode can be placed adaptively to the structure of an optical system used and yet can be heated very efficiently.
In this case, the semiconductor substrate preferably includes: a substrate region containing a dopant of a conductivity type opposite to that of the doped region; and a surface semiconductor layer of the same conductivity type as that of the substrate region. The semiconductor layer is preferably formed over the substrate region and has a dopant concentration lower than that of the substrate region. And the bottom of the recess preferably reaches the substrate region. In such an embodiment, the recess can be formed to a more accurate depth by taking advantage of difference in etch rate due to dopant concentrations.
In an alternative embodiment, the sub-mount may be a semiconductor substrate, and the heating member may be a polysilicon film formed on the substrate. Then, the heating member can also be provided easily at a low cost.
In yet another embodiment, the sub-mount may function as a photodetector with light-receiving areas.
In still another embodiment, the unit may further include: a mount for placing the sub-mount on a side face thereof; and a photodetector placed on the upper surface of the mount.
In still another embodiment, the unit may further include means for cooling the light-emitting diode down to such temperature as substantially eliminating kinks from a high-temperature I-L characteristic of the diode.
An inventive optical head apparatus includes: a semiconductor light-emitting diode; means for heating the diode up to such a temperature as substantially eliminating kinks from a low-temperature I-L characteristic of the diode; means for sensing the temperature of the diode; and a photodetector with light-receiving areas.
The optical head apparatus of the present invention with this arrangement can substantially eliminate kinks from the low-temperature I-L characteristic even at a low temperature.
An inventive optical disk system includes: a member for holding an information storage medium; a semiconductor light-emitting diode; means for heating the diode; means for sensing the temperature of the diode; a controller for controlling the heating means and getting the diode heated by the heating means up to such a temperature as substantially eliminating kinks from a low-temperature I-L characteristic of the diode if the temperature of the diode sensed by the sensing means falls within a range where the kinks are possibly caused in the low-temperature I-L characteristic; and a photodetector with light-receiving areas.
The optical disk system of the present invention with this arrangement can substantially eliminate kinks from the low-temperature I-L characteristic of the diode even at a low temperature.
In one embodiment of the present invention, the sensing means may sense the temperature of the diode from an operating current value of the diode corresponding to a predetermined emission intensity. In such an embodiment, the light-emitting diode can be heated quickly by a feed forward control.
In an alternative embodiment, the sensing means may also sense the temperature of the diode from a threshold current value of the diode.
In still another embodiment, the heating means is preferably incorporated with the photodetector. In such an arrangement, the optical disk system can be downsized and yet can heat the diode efficiently.
In yet another embodiment, the system preferably further includes means for cooling down the diode. The controller preferably controls the heating and cooling means such that the temperature of the diode falls within a range in which substantially no kinks are caused in the low- or high-temperature I-L characteristic of the diode.